1. Technical Field
The present invention relates to control valves in systems with parenteral fluid infusion. More specifically, the present invention relates to a valve which, although simple and inexpensive to manufacture, permits accurate and reliable adjustable settings at intravenous liquid flow rates.
2. Prior Art
In U.S. Pat. No. 4,332,369 to Gordon et al. there is disclosed an in-line valve for use in controlling flow of intravenous fluid. The valve employs a sluice-type gate member which is selectively reciprocated longitudinally in a control chamber by rotation of a disc member having a stem which threadedly engages a bore in the gate member. Reciprocation of the gate member varies communication between inlet and outlet passages of the chamber. The rotational position of the disc is selectively locked to preclude inadvertent changes of the valve setting by forcing a bearing surface radially against the disc periphery and snap-locking the bearing surface in place. The disclosure of the aforesaid U.S. Pat. No. 4,332,369 is expressly incorporated herein by reference in its entirety.
A modification of the valve disclosed in the aforesaid U.S. Pat. No. 4,332,369 is the subject of U.S. patent application Ser. No. 377,429, filed May 12, 1982 by Gordon et al. and entitled "Adjustable In-Line Valve for Sterile Fluids", the disclosure of which is also expressly incorporated herein by reference in its entirety. That modification involves providing a hermetic seal for the control chamber so as to avoid the potential for contamination of intravenous fluid through the gate member. Specifically, the disc is captured in an annular snap-fit in the control chamber so that the disc is free to rotate but not to move axially. The snap-fit seals the chamber from the ambient environment.
The gate member for the valve in the aforementioned Gordon et al. patent and patent application is of the sluice type having an arcuate edge or surface extending transversely of gate member movement and which is moved perpendicular to flow between the valve inlet and outlet passages across a controlled chamber. It has been found that, while these valves provide the desired locking feature and hermetic sealing, the flow control varies rapidly with respect to even small disc rotation throughout the control range of interest. More specifically, in administering parenteral fluid, the usual range of flow rate is from less than 1 to approximately 4 cc per minute. It is quite difficult to accurately vary flow rates within this range of interest with the valves described above. This can best be understood by envisioning a circular opening of the valve inlet passage into the control chamber and a sluice plate having a concave bottom edge being gradually moven to open or unblock more and more of the circular opening. The unblocked portion or flow area of the opening has a crescent or a convexo-concave configuration which varies in thickness as the gate member moves. It can be shown that this crescent-shaped flow area varies as a complex curve function of gate member displacement and provides very little control accuracy over the low flow rates (i.e. below 4 cc per minute) which are of interest in most parenteral liquid administration procedure.
One solution to the problem stated above would be to decrease the pitch in the threaded engagement between the valve stem and bore so that more turns of the disc are required to achieve a given change in flow rate. However, the valve must be able to quickly purge air from the intravenous line and to be quickly adjusted to deliver high flow rates in emergency conditions. If the stem and the bore thread pitch were decreased sufficiently to provide the desired control at low flow rate, the transition from low to high flow rate would require too many rotations of the disc to be useful in an emergency situation.